Various electronic components are incorporated in electronic devices. These electronic components are joined at predetermined positions on a substrate having a plurality of electrodes and lead frames, and are incorporated as such as a mounting structure in the devices. With the advancement of miniaturization of electronic devices in recent years, the electronic components incorporated in the devices are being more and more miniaturized, thus causing increased use of small-sized electronic components such as flip chips and chip size packages (CSPs) to be placed on a substrate.
Electronic components such as flip chips and CSPs have a principal surface on which a plurality of terminals are regularly arranged in an array, and each terminal has a solder bump formed thereon. In mounting such an electronic component on the substrate, the bumps are allowed to land on the electrodes on the substrate, called lands. Thereafter, the bumps are heated and melted (ref lowed), and then left to cool, so that the interconnection between the electronic component and the substrate is achieved. As a result, the terminals of the electronic component are electrically connected with the electrodes on the substrate, whereas the electronic component is held on the substrate via solder joints.
In addition to electronic components such as flip chips and CSPs, electronic components called chip resistors, chip LEDs, and chip capacitors are often mounted in mounting structures. Such electronic components are placed on electrodes on a substrate, after a paste containing metal particles (e.g., cream solder) is applied to the electrodes by a method such as screen printing. Thereafter, the metal particles are melted by reflowing and left to cool, whereby the electronic components are joined to the substrate. In general, the paste containing metal particles is applied to the electrodes on a substrate before electronic components such as flip chips and CSPs are placed on the substrate.
When thermal stress generated by thermal cycling, or external force, is applied to a mounting structure comprising a substrate and electronic components obtained through the aforementioned mounting process, and if the electronic components are joined to the substrate via the bumps, the solder joints may lack sufficient strength. As a countermeasure, a reinforcing resin is used to join the electronic components to the substrate, thereby to reinforce the solder joints.
One method of reinforcing the solder joints with a reinforcing resin is to allow an underfill material to enter gaps between the substrate and the principal surface of the electronic component having bumps thereon.
One proposal suggests a method of dispensing a reinforcing resin onto a substrate in advance before placing thereon an electronic component, only at positions which correspond to those on the peripheral edge portion of the electronic component (c.f., Patent Literature 1). This method, as compared with that of using an underfill material, is better in terms of facilitating repair work on the mounting structure.